You can know if consumers are dissatisfied, which is a sign of market failure. It's impossible to prove that harm doesn't exist, because you can't prove a negat

You can know if consumers are dissatisfied, which is a sign of market failure. It's impossible to prove that harm doesn't exist, because you can't prove a negative. If the monopoly does start substantially harming consumers, then I'd say that's when action should be taken, not before the market has a chance to respond.